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    • Thread Starter

    Could someone please explain rigid objects in equilibrium please, I have no idea how to do these questions. Example question:
    Q) A uniform rectangular picture ABCD, 3 metres wide and 2 metres high, has weight 400 newtons. It is supported on a trestle at a point E of DC, 1 metre from D, and kept upright by means of a rope attached to the picture at A. Calculate the tension in the rope and the normal and frictional forces at E.
    a) if the rope is horizontal
    b) if the rope is held at 40° to the vertical
    Picture below might make things a bit clearer. Thanks for any help (Rep goes to most helpful)

    a) 100N, 400N, 100N (I got the 400N)
    b)97.5N, 475N, 62.7N
    Thanks again

    for a)
    Weight left of E = 400/3 = 133+1/3
    W right of E = 2*400/3 = 266+2/3
    Take moments about E:
    2T + .5*133+1/3 = 1*266+2/3
    T = 100N
    Resolve vert:
    Normal = 400N
    Resolve horiz:
    Friction = 100N

    Hope thats clear enough

    taking moments around E
    anticlockwise moments = clockwise moments
    TN*2m = 0.5m *400N
    T = 100N

    400N is the normal reaction

    the friction at E is the force that will balance the whole system.
    the tension in the rope is providing a force of magnitude 100N to the left. the only force available to counter this is frction which will thus have an equal and opposite value: 100N to the right.

    is there a diagram for part b? tis a bit confusing
    • Thread Starter

    Thanks a lot, that makes sense. I just wasnt doing the moments properly because I wasnt sure how to take moments when the Tension was horizontal and the weight was vertical. But I guess its just a case of seeing the effect they have (ie horizontal tension pulls it A-clockwise and the weight pulls it clcokwise).
    As for the picture, sorry thats the only on in the book. What I think it means is that the rope is held upwards and the if the line DA was extended upwards then there would be an angle of 40° between the line and the rope. Ill let you know if I manage to get the answer.
    Thanks for the help
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Updated: June 21, 2005
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